Effectiveness of fluorine termination at nitrogen vacancies inside gallium nitride crystals based on first-principles calculations

Nitrogen (N) vacancies inside gallium nitride (GaN) crystals can scatter carriers and degrade the performance of GaN-based devices. Hydrogen (H) termination is an effective approach for eliminating defect levels in Si crystals but is less effective for GaN because the latter requires higher processing temperatures, which causes H to desorb more easily. Fluorine (F) is a potential alternative to H owing to its high chemical reactivity and small atomic radius. In this study, first-principles calculations were used to investigate the effectiveness of F termination at N vacancies in GaN crystals. The calculated density of states and band dispersion diagram indicated that F termination eliminated defect states near the conduction band edge and made the electronic states near the band edges resemble those of intrinsic GaN. These effects were attributed to the bonding of F atoms with Ga dangling bonds. Although H termination also resulted in the bonding of H atoms with Ga dangling bonds, the bonding states remained within the bandgap near the band edges; therefore, defect levels were not eliminated as effectively as with F termination. This behavior was attributed to the larger energy difference between the bonding and antibonding states of Ga-F bonds compared with Ga-H bonds. These results suggest that F termination can eliminate defect levels caused by N vacancies inside GaN crystals and improve the performance of GaN-based devices.